There are many applications that require precise flow control to ensure that a proper volume of liquid, gas, etc. is dispensed. Such applications include, for example, manufacturing, consumer products, medical systems, pharmaceuticals, etc. These various. applications require some type of process control that takes into account the flow rate of the process fluid or media and the amount of time that such fluid or media is flowing to dispense the proper volume. While many such applications control all aspects of the entire system, e.g., input pressure, fluid temperature, etc., other systems that have this same requirement are not so fortunate.
One such industry which requires volume control of the dispensed process fluid, but does not have the luxury of controlling all of the parameters that would affect such fluid flow, is the consumer appliance industry. For example, an automatic ice maker in the freezer compartment of a refrigerator includes a water control valve that is used to control the flow of water into the ice cube molds. As will be readily apparent, it is important that the flow of water into the ice cube molds does not continue for a period that would result in water overflowing the molds into the main freezer compartment. Likewise, the flow of water into the molds needs to continue for a period sufficient to fill these ice cube molds so that cubes of sufficient size are produced.
While typical process fluid control systems utilize a known, fixed input pressure and flow rate, the fact that consumer appliances are installed across the entire country in various municipalities and rural areas, no such controlled input pressure can be assured. That is, the input water pressure at one installation site in a particular town may be vastly different than the input water pressure in some other installation. Indeed, the input pressure may not even be maintained at a stable level during different periods of the day at a given location based on other activities that use the same water supply, for example, watering of the lawn, hydrant flushing, etc. Nonetheless, consumer appliances installed at all of these various locations are still expected to operate properly, i.e., fill the ice cube tray properly in this example.
One prior mechanism utilized in such systems employs a rubber flow washer that reacts to the varying input pressure conditions. The flow washers contain either a center orifice and/or bypass flow paths, both of which close down as the pressure acting on them increases. This is meant to control the flow through the flow washer to a known maximum level despite an increase in input pressure that would otherwise increase the flow rate. This flow washer acts to maintain the same delivered volume over time by restricting the orifice to clip or limit the flow rate increase that would otherwise occur. Depending on the flow washer, the typical flow curve rises somewhat linearly from 0 psi to about 20 psi and then flattens out to give a somewhat equal dispense over the remaining pressure range, typically to 120 psi. As such, normal operation would occur between about 20 psi to 120 psi, and a less than acceptable dispense would occur from about 20 psi and lower. Based on this maximum flow rate, the appliance controller simply opens the valve to the ice maker for a fixed period of time to fill the ice cube molds.
Unfortunately, the use of a flow washer does not provide acceptable results at lower input pressure conditions where the pressure is too small to affect any orifice size change on the flow washer. That is, while a flow washer can restrict or limit the maximum flow rate therethrough, it cannot maintain such a flow rate at low input pressure conditions. As a result, the timed operation of the valve used to fill the ice cube tray in this example may not provide an adequate volume of water at lower pressure installation locations. Consumers who have low water pressure, therefore, may become upset with the small size of the ice cubes, and may even incur the expense of a service call thinking that a malfunction has occurred in their ice maker. As is clear, then, such a flow washer is primarily a safeguard against an overfill condition, but may allow an underfill condition when the input pressure of the water line is low.
An alternate solution that has been employed in the appliance industry and elsewhere is the use of a flow meter in conjunction with the water valve to monitor the actual flow of water therethrough. A typical flow meter outputs a square wave pulse signal whose frequency is related to the flow rate of the media stream in which it is positioned. The use of such a flow meter should provide an accurate indication of the actual flow passing through the flow control valve and into the ice cube molds. Unfortunately, there is a change in the response of such flow meters based on the flow rate of the media itself. That is, at a given flow rate the flow meter will produce a certain amount of pulses per unit time. At a higher flow rate through the valve, the flow meter will give a different value of pulses per unit time. However, this change in frequency of the flow meter output is not linearly related to the change in flow rate itself. If it were, a controller would simply need to read the frequency of pulses to calculate the flow rate of the media, and then use this information to control the length of time that the valve is opened to dispense a given volume of fluid.
The non-linearity of the output of a flow meter is well documented and may be easily compensated by performing field calibration testing and adjusting the control parameters of the controller. However, such a requirement of individual field calibration of the flow meter in a mass produced consumer appliance is unworkable. Recognizing this, many consumer appliance manufacturers design their controls to prevent an overflow condition in high water pressure installation locations. They then simply have to accept the fact that underfill conditions may result because of the change in response of the flow meter at the varying flow rates caused by the differing input pressures at different locations and at different times throughout the day.
There exists, therefore, a need in the art for a process fluid control system that ensures a known volume of process fluid will be dispensed at each operation regardless of input pressure conditions at different locations and at different times throughout the day.